This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 100 21 669.2 filed in Germany on May 5, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to a fiber-optic current sensor having a reflection interferometer.
The invention relates to a fiber-optic current sensor having a reflection interferometer in accordance with the preamble of patent claim 1.
A generic fiber-optic current sensor is disclosed in DE-A-4 224 190 and G. Frosio et al., xe2x80x9cReciprocal reflection interferometer for a fiber-optic Faraday current sensorxe2x80x9d, Applied Optics, Vol. 33, No. 25, pages 6111-6122 (1994). It has a magneto-optically active sensor fiber which is wound in the form of a coil and surrounds an electric conductor. The sensor fiber is silvered at one end, and at the other end it is connected via a phase-retarding element to a polarization-maintaining optical supply lead fiber via which light can be launched into or outcoupled from the sensor fiber. In this arrangement, the supply lead fiber propagates orthogonally linearly polarized optical waves. Before entry into the sensor coil, the latter are converted into two circularly polarized waves with the aid of fiber-optic phase retarders, the two circularly polarized waves having a mutually opposite direction of rotation. After traversing the sensor coil, the two circular waves are reflected at the end of the coil, thereafter returning with exchanged directions of polarization through the coil.
If the current now flows through the electric conductor, the magnetic field of the current effects a differential phase shift between the two circular optical waves. This effect is termed the magneto-optic or Faraday effect. Owing to the twofold traversal of the coil, the waves accumulate a differential phase shift of xcex94"PHgr"s=4 V N I, V denoting the Verdet""s constant of the fiber, N the number of fiber turns in the coil, and I the current through the electric conductor.
Upon emerging from the coil in the phase retarders, the circular waves are to be converted into orthogonally linear polarized waves and guided to a detection system via the supply lead fibers. The phase shift caused by the current can be detected by causing the two reflected linearly polarized waves to interfere in a polarizer connected to the supply lead fiber.
In order to obtain a finer resolution in the detection of the differential phase shift, the effective operating point of the interferometer must be situated in a linear range of its cosinusoidal interference function. The differential phase of the two interfering linearly polarized waves is modulated in order to achieve this. The supply lead fiber is operationally connected to a modulator for this purpose. A piezoelectric ceramic operated at resonance and around which a few turns of the supply lead fiber are wound is generally used as modulator. The modulator modulates the birefringence of the fiber and thus the differential phase of the two waves. The frequency of the modulation is typically in the range of 100 kHz and a few MHz and is determined, inter alia, by the length of the fiber connection at the sensor fiber, that is to say the supply lead fiber.
However, it is difficult in practice to use a piezoelectric ceramic to modulate the differential phase of two orthogonal optical waves with a sufficiently large amplitude. In the prior art, recourse is therefore made to a combination of measures in order to achieve the desired amplitude. Thus, in the region of the modulator the supply lead fiber, generally provided with an elliptical core, is replaced by a section of a more sensitive fiber with the stress-induced birefringence. A high modulator voltage is used, and a hollow cylinder is employed as piezoelectric ceramic instead of a disk-shaped element. However, these measures lead to various disadvantages: thus, more sensitive fibers with stress-induced birefringence are more dependent on temperature, are not widely available on the market, are expensive and, moreover, are in some ways difficult to splice with other types of fiber. Again, a fiber with stress-induced birefringence constitutes an additional component in the sensor, and this increases the complexity of the design. The high modulator voltage leads to a strong mechanical loading of the ceramic and thereby impairs the stability and the service life. Finally, the hollow cylindrical ceramic has a lower resonant frequency than the disk-shaped one, and this results in a lower useful bandwidth of the sensor. Moreover, a longer supply lead fiber has to be used, since the length of the modulation frequency must be matched.
A fiber-optic current sensor having another interferometer, a so-called Sagnac interferometer is known from G. Frosio et al., xe2x80x9cAll-fiber Sagnac Current Sensorxe2x80x9d, Proc. Opto 92, pages 560-564 (1992) and EP-A-0 856 737. In the Sagnac interferometer, two oppositely directed light waves are propagated in a closed optical circuit. The two waves are polarized circularly in the sensor coil and linearly in the two connecting fibers of the coil. The linear polarizations are aligned parallel to one another in this case. By comparison with orthogonally polarized waves, the modulation of the differential phase of two oppositely directed waves with parallel polarization requires 100 to 1000 times less piezoelectric deformation of the modulator, and so that above-named disadvantages of the reflection interferometer are not present. The sensor with the Sagnac interferometer has the disadvantage, however, that it is vulnerable to mechanical vibrations. This is due, inter alia, to the finite propagation time of the waves in the optical circuits, since the two waves reach the location of a disturbance at different times, and to the inherent sensitivity of the Sagnac interferometer to rotational movements.
The phase modulation of the oppositely directed waves in the Sagnac current sensor is performed in an entirely analogous fashion to the modulation of the oppositely directed waves in a fiber gyro for measuring rotational speeds, such as described in H. C. Lefevre, xe2x80x9cfiber-optic gyroscopesxe2x80x9d, Fiber-Optic Sensors, J. Dakin and B. Culshaw Editors, Vol. 2, Chapter 11, Artech House 1989.
It is the object of the invention to create a fiber-optic current sensor having a reflection interferometer of the type mentioned at the beginning which can be modulated in a simple way.
The differential phase of two oppositely directed, parallel linear polarized waves is modulated in the current sensor according to the invention. In order to permit this, a section of a fiber-optic supply lead of the current sensor according to the invention has two fiber arms, the two fiber arms interconnecting two fiber couplers. Propagating in the two fiber arms are linearly polarized waves which are converted into orthogonal polarizations in one of the couplers before they reach the coil-shaped optical sensor element. The orthogonal polarizations returning from the sensor coil are once again split in the second coupler between the two fiber arms and reunited in the first coupler. Means are present for changing the direction of polarization with respect to the fiber axes in one of the fiber arms. The modulation is performed in one or both of the fiber arms.
Since the separation of the fiber-optic supply lead takes place only over a short distance, the signal is virtually uninfluenced by mechanical vibrations.
Waves with linear polarization can propagate in the two fiber arms parallel to the long and short axes of the fiber core. However, at least one, and in a selected embodiment even both, fiber arms preferably have a polarizer such that only a single direction of polarization is present in the fiber arm.
The means for changing the direction of polarization with reference to the axes of the fiber core is preferably a 90xc2x0 splice which connects two fiber segments in one of the two fiber arms.
In a preferred embodiment, a detector and a light source are connected to the same fiber segment of a fiber coupler. This arrangement is suitable, in particular, for measuring direct currents.
The current sensor according to the invention can be multiplexed in a simple way by supplying a plurality of coil-shaped sensor fibers from the same fiber-optic supply lead. For this purpose, the fiber-optic supply lead has an appropriate number of further fiber arms, which are arranged in series with the first two fiber arms.